1. Field of the Invention
The present invention relates to a RAID (Redundant Arrays of Inexpensive Disks) apparatus which allocates a plurality of same logical volumes to a plurality of physical disk units and an access control method therefor, and, more particularly, to a RAID apparatus which prevents access requests and an access unbalanced control method therefor.
2. Description of the Related Art
Disk storage systems such as a magnetic disk system are used as an external storage system in a computer system. A host computer accesses such a disk storage system with a logical volume name that an OS (Operating System) recognizes. Logical volumes are allocated in a disk storage system.
If one set of individual logical volumes is allocated in such a disk storage system, when a physical disk unit where some of the logical volumes are located fails, those logical volumes cannot be used any more.
To prevent this problem, a RAID apparatus has been proposed. A RAID apparatus has a plurality of same logical volumes allocated on different disk units. When one disk unit fails, another disk unit where the same logical volume of interest is allocated is used. This system can prevent the occurrence of an event that any logical volume becomes unusable due to failure of the associated disk unit.
FIG. 8 is an explanatory diagram of prior art.
As shown in FIG. 8, a RAID apparatus comprises a plurality of magnetic disk units 91-1 to 91-4 and a disk controller 90 which controls those disk units. FIG. 8 shows the RAID apparatus with a mirror structure which includes four magnetic disk units 91-1 to 91-4.
A logical volume LM0 is allocated on the magnetic disk unit 91-1. The same logical volume LM0 as located on the magnetic disk unit 91-1 is allocated on the magnetic disk unit 91-2. A logical volume LM1 is allocated on the magnetic disk unit 91-3. The same logical volume LM1 as located on the magnetic disk unit 91-3 is allocated on the magnetic disk unit 91-4.
Even if the magnetic disk unit 91-1 fails, the logical volume LM0 can be accessed by using the magnetic disk unit 91-2. Even if the magnetic disk unit 91-3 fails, likewise, the logical volume LM1 can be accessed by using the magnetic disk unit 91-4.
When a plurality of logical volumes are set in one physical disk unit, however, a high-rank apparatus, such as a host computer, issues two or more access requests to the same physical volume. In such a case, since one physical disk unit cannot execute two operations at a time, the prior art scheme suffers the inherent problem that some access requests should wait.
The operation of the magnetic disk units takes a relatively longer time than the operation time of a high-rank apparatus. If some high-rank apparatus frequently issues an access request to the same logical volume, therefore, the number of access request that should wait increases. This increases the time from the issuance of an access request to the end of the requested operation, thus reducing the system access speed.